The present invention relates to a drive force transmission device that transmits drive force through frictional engagement between drive force transmitting members that are coaxially arranged in a manner rotatable relative to each other.
To improve performance, a drive force transmission device may typically include a plurality of fine grooves formed in a sliding surface of a drive force transmitting member. For example, as described in Japanese Laid-Open Patent Publication No. 2005-36863, a plurality of fine grooves are formed in a sliding surface of one of a drive clutch plate and a driven clutch plate, which form a drive force transmitting member of a drive force transmission device. The grooves extend in parallel with one another in the circumferential direction of the sliding surface. The grooves maintain an appropriate thickness of an oil film formed between the opposing sliding surfaces of the drive and driven clutch plates. In this manner, the sliding surfaces exhibit μ-v characteristics of high friction and positive gradient, which indicates that the friction coefficient μ increases as the sliding speed v increases. Accordingly, the drive force transmission device reliably obtains desired drive force transmission characteristics with improved anti-judder properties.
The sliding surfaces of the drive force transmission device must maintain the desired drive force transmission characteristics even after a certain period of use, or an assumed length of life of the drive force transmission device. Specifically, regardless of wear of the sliding surfaces caused by long-term use, the shape of each of the fine grooves must be maintained in an appropriate state, or an oil film formed between the sliding surfaces must be maintained in a desired state. It is thus ideal that such fine grooves are formed to have rectangular cross-sectional shapes. However, since the fine grooves of such shapes are difficult to form, typical fine grooves have substantially triangular cross-sectional shapes. In other words, the width of each fine groove decreases from the opening end of the groove toward the bottom. If the fine grooves are formed through cutting as in the case of the aforementioned document, the triangular cross-sectional shape of each fine groove becomes more acute. Thus, as the sliding surfaces become worn due to the long-term use, the width of each groove becomes gradually smaller, changing the drive force transmission characteristics. To avoid this problem, each fine groove is formed initially with an increased width at an opening end and an increased depth, in such a manner that a sufficient groove width is maintained regardless of the wear of the sliding surfaces. Therefore, in a typical drive force transmission device, the sliding area (corresponding to the portions other than those corresponding to the grooves) of the sliding surface in which the fine grooves are formed is approximately 50% or smaller of the total area of the sliding surface at the initial stage of use.
However, if each fine groove has an increased width at the opening end, the sliding area of the sliding surface becomes excessively small particularly at the initial stage of use, thus increasing surface pressure. This causes a significant decrease of the width of each fine groove when the sliding surface becomes worn. The drive force transmission characteristics thus greatly change after a long time of use, compared to the initial stage of use.